The invention pertains to a safety valve for a consumer operated by compressed air.
A safety valve of this type is described, for example in DE 3,005,547 C2 and in DE 196-22,198 A1.
Safety valves of this type are used, for example, to actuate the clutches and brakes of presses. In the case of a switching malfunction in which the two directional control valves assume different switching positions, the feed in these types of safety valves is blocked, and the line leading to the consumer is vented, so that no residual pressure remains in this line. To determine a switching malfunction of this kind and to turn off the system, it is provided in the safety valve described in DE 3,005,547 C2 that the control air for the pilot valves is taken not directly from the feed line but rather from cross-channels, which connect the two bores in which the valve disks are guided to each other in a crosswise manner. As a result, it is possible for the safety valve to monitor itself dynamically at all times.
The task of the invention is to improve a safety valve of the general type in question in such a way that, after the malfunction has been corrected, the valve can be reset or turned back on easily, preferably without the need for electric switching elements.
The task is accomplished by a safety valve for compressed air-operated consumers with two directional control valves connected in parallel, each with its own working piston and its own valve head connected to the piston. Each of the directional control valves is switchable by its own pilot control valve. Each of the two valve heads is guided in a bore in a valve housing. The bores are connected crosswise to each other by two cross channels. The valve seat of the first pilot valve is connected via a pilot channel to one of the cross channels which proceeds from the bore of the valve head. The valve seat of the second pilot valve is connected via a second pilot channel to the other cross channel which proceeds from the bore of the other valve head. A compressed-air switching element is provided between two corresponding points at each of the two directional control valves, by means of which switching elements the safety valve can be blocked when the pressure at these two points are different and turn back on again only by the external compressed air-operated actuation of at least one of the two switching elements. The present invention offers not only the advantage that the safety valve can be blocked when a problem occurs in one of the two directional control valves but also advantage that the valve can be turned back on again by external actuation of the switching element, which can be done by the use of a key-operated switch, i.e., manually, or by shutting off the system pressure completely.
Thus, for example, it is especially advantageous that each switching element has a device for dynamically monitoring the pressure differences in at least two pressure lines, each of these devices being provided with a piston-cylinder unit, which, when there is a difference between the pressures in the two pressure lines, connects the inlet of one of the two directional control valve to the atmosphere. Connecting the inlet of one of the two directional control valves to the atmosphere has the result of preventing the unintentional and unwanted re-actuation of the safety valve.
There is preferably a switching element in each of the pilot channels.